(1) Field of the Invention
This invention relates to a photographic apparatus such as photoengraving camera or the like, and more particularly to a photographic apparatus permitting automatic control of optimum exposure conditions by means of a microcomputer or the like, as well as to an exposure controlling method.
(2) Description of the Prior Art
Photoengraving cameras are used to take continuous tone pictures and pictures of line subjects, each of which requires a single exposure, and halftone pictures which need individually an auxiliary exposure such as flash exposure or the like in addition to a main exposure. Whichever picture is taken, it is unable to obtain a reproduced picture of good quality unless suitable exposure conditions are set up.
Upon setting up exposure conditions, the optimum values of the light source intensity, exposure time and iris value were determined for many years by individual operator's experiences while taking the density of a given copy and the desired photographic magnification into consideration. However, a variety of automatic exposure control systems have been developed in recent years.
Such conventional automatic exposure controlling methods may be divided roughly into two types, namely, (1) the copy density measurement type and (2) the focus-point photometric type.
In the copy density measurement type (1), magnification and iris value are first of all set up usually in addition to the density of a copy. Then, a calculation is made on the basis of these data to determine a suitable exposure time.
The copy density measurement type (1) features as its advantage that it can readily follow magnification variations and copy density changes over a broad range. On the other hand, it is accompanied by many problems for performing automatic exposure control with good accuracy that it requires a separate densitometer to measure the densities of the high-light point and shadow point of each copy; it is indispensable, when the reciprocity law failure has occurred, to perform complex calculations and processings including changes of reciprocity law failure to correct the reciprocity law failure; and it must follow variations in illuminance distribution on each copy and changes in basic data to be developed by deterioration of light sources or lamp replacement.
On the other hand, the focus-point photometric method (2) may be practiced by way of example by choosing the exposure time first of all and then by measuring brightness such as the high-light point and shadow point on a picture placed on the focus point to adjust the iris value and the intensity of each light source. It does not require any densitometer. Accordingly, it is not necessary to go through with such work as the setting of a density.
It can also control an important point on the focus point such as the high-light point by means of a sensor to keep the brightness of the important point at a constant level. Therefore, there are such merits that it permits to hold the exposure time at a constant level and it does not require to take the reciprocity law failure into consideration.
However, the light quantity adjustment term is generally centered at the iris value. Thus, the focus-point photometric method (2) is accompanied by a drawback that the adjustable range of the iris value is limited and no optimum iris value may be set for the reproduction of a picture of good resolving power.
Furthermore, a light sensor is required to show good reproducibility as to its sensitivity upon measurement of brightness. A light sensor having an extremely high level of sensitivity is required for the measurement of the brightness of each shadow point.
In the focus-point photometric method (2), a sensor is in its entirety housed in a hand stand (camera bag). It is thus difficult to practice the focus-point photometric method (2) with a roll film type camera which imposes limitation on work by the hand stand.
Both of the above-mentioned conventional methods (1) and (2) are accompanied by such inconvenience that an operator is required to find out the high-light point and shadow point from a picture on a copy or the picture image of the copy projected on the focus point and then to measure the density at each of the points.
According to the second embodiment of the focus-point photometric method (2), the exposure time may be controlled in accordance with the brightness of an image on the focus point without adjusting the iris value. This method is an intermediary method of the above-described two methods and is still accompanied by one or more of the drawbacks of the two methods.